Associative Memory

An associative memory can be considered as a memory unit whose stored data can be identified for access by the content of the data itself rather than by an address or memory location.

Associative memory is often referred to as Content Addressable Memory (CAM).

When a write operation is performed on associative memory, no address or memory location is given to the word. The memory itself is capable of finding an empty unused location to store the word.

On the other hand, when the word is to be read from an associative memory, the content of the word, or part of the word, is specified. The words which match the specified content are located by the memory and are marked for reading.

The following diagram shows the block representation of an Associative memory.

From the block diagram, we can say that an associative memory consists of a memory array and logic for ‘m’ words with ‘n’ bits per word.

The functional registers like the argument register A and key register K each have n bits, one for each bit of a word. The match register M consists of m bits, one for each memory word.

The words which are kept in the memory are compared in parallel with the content of the argument register.

The key register (K) provides a mask for choosing a particular field or key in the argument word. If the key register contains a binary value of all 1’s, then the entire argument is compared with each memory word. Otherwise, only those bits in the argument that have 1’s in their corresponding position of the key register are compared. Thus, the key provides a mask for identifying a piece of information which specifies how the reference to memory is made.

The following diagram can represent the relation between the memory array and the external registers in an associative memory.

The cells present inside the memory array are marked by the letter C with two subscripts. The first subscript gives the word number and the second specifies the bit position in the word. For instance, the cell C_ij is the cell for bit j in word i.

A bit A_j in the argument register is compared with all the bits in column j of the array provided that K_j = 1. This process is done for all columns j = 1, 2, 3……, n.

If a match occurs between all the unmasked bits of the argument and the bits in word i, the corresponding bit M_i in the match register is set to 1. If one or more unmasked bits of the argument and the word do not match, M_i is cleared to 0.

Cache memory Organization

Cache Memory is a special very high-speed memory. It is used to speed up and synchronizing with high-speed CPU. Cache memory is costlier than main memory or disk memory but economical than CPU registers. Cache memory is an extremely fast memory type that acts as a buffer between RAM and the CPU. It holds frequently requested data and instructions so that they are immediately available to the CPU when needed.

Cache memory is used to reduce the average time to access data from the Main memory. The cache is a smaller and faster memory which stores copies of the data from frequently used main memory locations. There are various different independent caches in a CPU, which store instructions and data.

Cache Performance:
When the processor needs to read or write a location in main memory, it first checks for a corresponding entry in the cache.

• If the processor finds that the memory location is in the cache, a cache hit has occurred and data is read from cache
• If the processor does not find the memory location in the cache, a cache miss has occurred. For a cache miss, the cache allocates a new entry and copies in data from main memory, then the request is fulfilled from the contents of the cache.

The performance of cache memory is frequently measured in terms of a quantity called Hit ratio.

Hit ratio = hit / (hit + miss) =  no. of hits/total accesses

We can improve Cache performance using higher cache block size, higher associativity, reduce miss rate, reduce miss penalty, and reduce Reduce the time to hit in the cache.

Cache Mapping:
There are three different types of mapping used for the purpose of cache memory which are as follows: Direct mapping, Associative mapping, and Set-Associative mapping. These are explained below.

1. Direct Mapping –
   The simplest technique, known as direct mapping, maps each block of main memory into only one possible cache line. or
   In Direct mapping, assigne each memory block to a specific line in the cache. If a line is previously taken up by a memory block when a new block needs to be loaded, the old block is trashed. An address space is split into two parts index field and a tag field. The cache is used to store the tag field whereas the rest is stored in the main memory. Direct mapping`s performance is directly proportional to the Hit ratio.For purposes of cache access, each main memory address can be viewed as consisting of three fields. The least significant w bits identify a unique word or byte within a block of main memory. In most contemporary machines, the address is at the byte level. The remaining s bits specify one of the 2^s blocks of main memory. The cache logic interprets these s bits as a tag of s-r bits (most significant portion) and a line field of r bits. This latter field identifies one of the m=2^r lines of the cache.

Application of Cache Memory –

1. Usually, the cache memory can store a reasonable number of blocks at any given time, but this number is small compared to the total number of blocks in the main memory.
2. The correspondence between the main memory blocks and those in the cache is specified by a mapping function.